Hi! Here are some Quick How-Tos :)
(sorry I'm no help with other image-editing software besides GIMP)
VERSION 1 Editing a Glass TXTR
In GIMP, ctrl-n to start a new file (size 64x64 px)
You should have a tiny white square
In the Layers-Brushes window/port, right-click your Background layer and select Add Alpha Channel
Toolbar > Colors > Color to Alpha...
You should have a tiny clear square
In the new window, click OK and ctrl-shift-e to export as a png, name it glass or whatever you like
In SimPe, open your window or door and select the Texture Image (TXTR) in the Resource Tree and find the glass TXTR in the ResourceList.
In Plugin View, right-click on the image to Build DXT...
In the DDS Builder window, add your clear png in the open Image...
Sharpen: None and Format: DXT5Format in Settings (if it isn't already) and click Build
Click Commit and ctrl-s to save
Load your game and see how you like it
VERSION 2 Editing a Too-Dark Glass TXTR
I'm using platasp_GP07_ArchedAperture from the Strangerville Pack
In SimPe, select the Texture Image (TXTR) in the Resource Tree and the glass TXTR in the ResourceList (e.g. archedaperture-glass-base_txtr). In Plugin View, right click the texture and Export...
In GIMP, open the png (e.g. archedaperture-glass-base_txtr_256x256)
Toolbar > Colors > Brightness-Contrast...
In the new window, make sure Preview is ticked and then adjust the Brightness slider: somewhere in the 50-70 range looks pretty good in this case. Click OK
Toolbar > Colors > Color to Alpha...
In the new window, click OK
The png should still have ~just a little~ shading
ctrl-shift-e to export, I usually add a 2 to the file name (e.g. archedaperture-glass2-base_txtr_256x256)
Back in SimPe, right-click on the image to Build DXT...
In the DDS Builder window, add the new png in the open Image...
Sharpen: None in Settings, click Build
Click Commit and ctrl-s to save.
Load your game and see how you like it
Good luck and happy simming! :)