Finder is renaming my photos

Hello! I have this recurring frustrating problem. A photograph I have in finder get's renamed and instead of the name I put to it, it shows with this ____~866 as name. When I try to rename it with it's original name, finder tells me I can't because a photo already exists with this name. Only it appears like this ____~866.... If I manage to change this by changing the name a bit, another photo get's renamed and it starts all over again...
Does anyone know what can I do about this and what does it mean?