I don't know if it's a bug, really. Anyway, I activated the tracking on a MySQL table which contains an image media and I noticed two strange things:
1) the changes are not visible in the log table until I exit and reopen the solution
2) If I change something, let's say, in the First Name field, I get two records in the log: one for First Name and one for the ImageMedia field, which hasn't been touched.