I would say for your Guild Screen shots that I seen you do, you probably could have named it like SS-Guild1.jpg. This would say it is a screen shot of Guild 1, for guild creation maybe SS-Guild_Creation.jpg, if more then one used then number them based on the steps.

But this does not really matter what Icy is mainly referring to is the Skills/Icons/Items stuff. After all you have a Scroll Image for the Skill you buy, Icon for the Skill you use and they both have the same name. In other words, prefixed files not only make them searchable as a set, they are also distinguished for their appropriate contexts.

The prefixes that I know of so far are as follows:
Item - refers to items in the game, Armor, weapons, loot, etc... Basically anything you can house in your inventory. Can have suffix SS to tell if it is an In game Screen shot of the item.
Skill - refers to Skills. If it is on your Quick Bar as a skill to effect target then it would be referred to as a Skill.
Icon - Icons are images that reflect other things. They are used for like Face Icon for NPCs/Mobs, anything that has 2 pictures to represent it and one is much smaller then the other. Smallest will be Icon. Also used for Map Icons.
Monster - refers to the large image of a monster.
NPC - refers to the large image of a NPC
Map - Image of a Zone. Suffix - Lvl - refers to the map that has the levels of monsters listed.
Coin - In game currency images.
SS - In game Screen shot of certain screens. If of an item please consider Item as the prefix and SS as the suffix.
As for everything else - a succinct and sensible name will suffice

I'm sure there are more but I can not think of them for now.

Explanation:
Image: prefix-name-suffix.type

Example:
Image:Map-Roumen-Lvl.png

Icy, if you would please correct anything that I am missing or mistaken on. Or make a nice list for us to see what all the options are :D