The Art Gallery can be found in the collection tab of the main menu.
To unlock everything you need a total of 7.100 credits (3.400 for the first row, 3.000 for the second one and 700 for the last one).
There is no easy way to get a lot of credits. Credit drop depends on your score and on if you have beaten/improved your previous rank.
Infinity Drive seems to have a good drop rate at higher levels.