I did my degree in photography and a major part of the course was about copywrite. It is not so confusing as you have all made out. If someone is exactly duplicating/reproducing your photo for financial gain whilst passing it off as their own or not acknowledging you as the creator then its clearly a breach of copy write. Whether there is any financial gain even once acknowledged is still more of a moral and personal issue for the photographer who should endeavour be aware of their own value from the start and be very careful how and where they publicise their art. It could be said that the more amature photographers that give work away for free may in turn devalue the work of professionals. This may be true. However, in small publications which help an amature to become known when a professional would insist on some over rated charges, it is understandable that an author might decide to ask for "donations". It is up to the individual photographer whether they prefer the recognition over the cost and to live with the consequences. There are many ways to protect your work and it is not difficult to seek legal contractual advise if you are not sure of long term consequences.
Things get a little more tricky when the format is altered - think of Andy Warhol, pop art as the prime example- he took well known images and changed the meaning by the multiplying pictures of popular icons using screen printing. He would argue that his work had altered the meaning of the original image and by use of a different medium and so in doing had created a kind of narrative/commentary of his own about the original images. Therefore creating a juxtaposition which in turn made his art original. Post modernism plays on these techniques and themes all the time. Manipulation of an image may not necessarily be fair but what morally right is some what different however. Once you have put your work on a public domain, without a copy write clause or preventative lock on your original file, be prepared to have your work used by others. This may seem unfair and even wrong but you should protect your self from this from the start. It is a lot harder to fight the big boys, even when you know its your art, proving it is often expensive and often does not mean great rewards unless you are some big nose like MacDonald's. More and more artists are using photoshop and the likes for this - I would just advise rather than risk any litigation to use your own images where possible, however montages etc that alter the meaning and become an image in their own right have been used to great effect in the art world without issue. If a drawing is created from a photograph it is almost impossible to say that the artist has not made an interpretation by use of another medium, I have seen so many drawing styles derived from one photo image in an art class, however this is still more of a moral issue rather than a copy write issue about the integrity of the artists work than anything else.
I used to work as a printer, we had a picture of a dog in the window- it was my colleagues dog and we had the negative. A lady came in and insisted we had used her photo of her dog for profit, we had not but she insisted and took her case to a lawyer. We proved our case in our favour and she wasted a lot of her own money and our energy, our evidence was the negative, although her picture was of another Alsatian dog it was clearly different although similarly posed etc. It goes to show many images can be alike yet with the original negatives together it was clear they were not the same dog. Some caving images can also be similar, but you cannot have claim on this. However it goes to show as original as you think you are being don't be too surprised if someone has done it before. This does not affect your own copywrite as long as you keep raw files and negatives as proof and any other cooberating evidence no one can claim it as theirs and visa versa. I have seen photos taken 20 years apart that appear the same but as both photographers were working independently and with out knowledge of one another and had their negative evidence of their work then neither could claim stealing of the others ideas. Proving original thought processes is where it may become less clear. It is rare when was any idea completely original. Photographs can easily end up very similar without the intention of copying, just compare family photo albums with friend its not hard to see similarities.
You can also sign your rights away as a photographer if it is work undertaken as a commission or whilst working for someone. You would receive a one off agreed payment and the negatives then would belong your employer for this work and duration.
So to summarise, know the value of your art and be responsible and protect it from the start. Financial gain may not be the only benefit to having your work published and not immoral if done in a way that all respective parties have agreed. The question is how you expect to benefit from work published- now and in the future? Most of the people I have known including Wolfgan Tilsman rarely pursue copywrite claims even with their fame much of it is free advertising and rarely amounts to much unless it is out and out forgery.