Every screenshot from a server running version < 1.7 will be rejected. This has two reasons. Firstly it's easier to use skins in a previous version. Secondly there are lots of bugs in old versions which might lead to false judgements.

Screenshots from servers running the latest version (actually 1.7) will be accepted and processed as usual. If they show a clear advantage (compared to the normal gameplay), it leads to a ban. This includes bright colours, oversimplified textures or changed iron sights, for example. Every ticket is an individual case assessment.

Background:
Serveradmins choose to accept or deny modified game clients by setting the dvar sv_pure. If it's set to 1, only unmodified clients are able to connect, others will be kicked. This function works properly since patch 1.3. Sadly version 1.7 has a bug which still allows players to modify their textures although it's not the intention of the developers (=cheating).

Please consider this before opening tickets containing screenshots as evidence.

Your GGC-Stream Staff