Bitbucket

The Image comparison tool is very nice. You can view old and new images side by side, merged (whole image by percentage or with slider) and pixel differences.

The price structure is a trap. 10 users are very cheap (USD 10 pa?) and starting with the 11th user it gets expensive (USD 1800 pa?). Still not very expensive, but the jump is huge.

Can invite multiple reviewers.

Flexible work flow

Gitlab

Only one reviewer