I had a problem that looked similar.
The problem wasn't loose belts. The belts were tight and seemed fine. I had a problem where the connection from the belt to the moving parts wasn't as secure as it should have been, allowing for some slop in the movement, which resulted in circles that looked much like yours.
In my case, the whole design of the printer was crap, and it needed a major rebuild. There was simply no way to get a good connection between the belt and the Y bed movement, or the X carriage movement at the point where it was at.
I would suggest that you look at the screws that connect the belt holder to the bed, and the ones that connect the belt to the print head carriage.
MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.
Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts
