"Sanatçı" is a general noun for those who works in the field of Art. It can be a singer/painter/musician/photographer/actor/dancer etc. However "ressam" is used only for "painter". A "ressam" is also a "sanatçı. However, a "sanatçı" does not have to be a "ressam".
"Sanatçı" is originated from the word "sanat", which means "art". "Ressam", which is a borrowed word, is originated from "resim", which means "drawing, picture". It can only be used for painter.
PS: In turkish, we generally produce new words by adding suffixes at the end, for example; "sanat → sanatçı". We do not change the root of the word, like above example "resim → ressam".
Very good Q. Im not fluent but as someone w. partial Azeri Turkish ancestry, i'm confident -im at end of such noun or adjective means "I AM" whatever noun or adjective came b4 -even though there'e already Ben in this case. The one about the BIR was also very astute Question...i think you may only use it to emphasize SINGULARITY and it may be unnecessary since BEN already implies singularity and luckily among the Sultans centuries of blood fueds, one positive result was the language evolving practical elements such as reducing redundant/extra unneeded parts when something is already implied or obvious. Good luck. In hindsight, you might think although the concept of I/Me is mentioned twice in both Ben and -Im, Ben is still kinda helpful in establishing singularity of noun Resam/Painter.
Thank you so much for your reply. So even if i use biz or onlar i can still use ressam instead of ressamlar to mean painters. Because in both english and arabic we say we are painters, i an a painter or انا رسام نحن رسامين (ana rassam nahno rassamin). So biz ressamiz (i without a dot) means we are painters even if it is ressam.