Ok, for those of you who have tried to use a spray in TF2 and realised it doesn't work, here is a simple method which I'm using to import my own sprays:
1. Find your image, it must be square the following sizes are acceptable: 64*64 128*128 256*256.
2. Download VTFedit.
3. With VTFedit, Import your own .JPG,.BMP, etc... and convert it to a .VTF file (Valve Texture File)
4. Now place the .VTF file in:
(your dir)\Steam\steamapps\(your username)\team fortress 2\tf\materials\VGUI\logos
5. To fix the quick view appearing blank copy the same .VTF also in:
(your dir)\Steam\steamapps\(your username)\team fortress 2\tf\materials\VGUI\logos\UI
6. In game, go to the options and click import the spray and browse for it in the \logos folder (not the UI folder!) and validate. (this will create a VMT file near your VTF)
In summary; your .VTF must reside in the last \logos and also \UI folder where you copied it manually, then don't forget to do the step 5, the spray now appear correctly in the list, I have tested to spray it in game and it works fine.
If you have any problems please let me know in my profile.