After having never used a trackball, I recently purchased a Logitech Trackman Wheel Optical
which is similar in design to Logitech's M570
. I have a beef with wireless stuff otherwise I would have got the M590. I really like moving the trackball with my thumb - the movement feels very natural to me. It took about 10 minutes to get used to and using the driver to make it move more responsively improved my experience further. After a single day of using it I temporarily had to go back to a regular mouse for a day and I was miserable.
I have noticed that precision tasks such as photo touchups are not something that I can do with this mouse. If I want to move the cursor only slightly in one direction, it ends up going too far over. This problem does not occur in my normal web browsing experience. It's probable that if I really had to I could slow down the responsiveness of the curser and gain sufficient precision that way, but because that would have to be changed back for efficient browsing most likely I'll just switch to a different mouse when photo editing. Since you mentioned graphics I thought this was important to note.
If you can, it'd be wise to consider visiting a computer store so you can feel different trackballs and see if one stands out as remarkably comfortable in your hand and intuitive with movement.