Here is the full list of all 99 Pixelarium achievements.
350 S Finished
More than 145 colors
More than 185 colors
More than 210 colors
10 M Finished
Bigger than 15k pixels
Bigger than 20k pixels
Bigger than 30k pixels
Bigger than 45k pixels
Bigger than 65k pixels
Bigger than 80k pixels
Bigger than 110k pixels
1 L Finished
4M pixels painted
Bigger than 12k pixels
1 M Finished
200 S Finished
100 S Finished
More than 110 colors
50 S Finished
2M pixels painted
More than 80 colors
25 S Finished
Bigger than 10k pixels
More than 65 colors
10 S Finished
Bigger than 6k pixels
Bigger than 7k pixels
Bigger than 8k pixels
Bigger than 9k pixels
1000 XS Finished
500 XS Finished
1M pixels painted
200 XS Finished
100 XS Finished
500k pixels painted
50 XS Finished
25 XS Finished
10 XS Finished
850 XXS Finished
300k pixels painted
550 XXS Finished
200k pixels painted
150k pixels painted
350 XXS Finished
More than 45 colors
100k pixels painted
200 XXS Finished
70k pixels painted
100 XXS Finished
40k pixels painted
More than 30 colors
50 XXS Finished
20k pixels painted
25 XXS Finished
10 XXS Finished
10k pixels painted
More than 20 colors
1 XXS Finished
Bigger than 2k pixels
Bigger than 3k pixels
Bigger than 4k pixels
Bigger than 5k pixels
1 S Finished
5k pixels painted
2k pixels painted
Bigger than 1k pixels
More than 5 colors
More than 10 colors
1k pixels painted
1 XS Finished
Bigger than 140k pixels
Bigger than 170k pixels
Bigger than 210k pixels
Bigger than 260k pixels
7M pixels painted
10M pixels painted
15M pixels painted
20M pixels painted
40M pixels painted
70M pixels painted
10 L Finished
25 L Finished
50 L Finished
100 L Finished
200 L Finished
25 M Finished
50 M Finished
100 M Finished
200 M Finished
350 M Finished
600 M Finished
550 S Finished
800 S Finished
1 XL Finished
4 XL Finished
9 XL Finished
16 XL Finished
25 XL Finished