If i remember right its just a generic progress bar but the 'fill' image is a shiny blue. I think the border is custom but again im recalling from memory.
The way Ive been doing progress bars is...
1) create a dialog box with a custom background image. 2) create a dialog image with the actual image value being what you want to fill your bar. Make sure the size in X of this image is set to the size or a bit smaller than you dialog box. 3) place the dialog image in the dialog box anchored to the left.
To show progress you simple offset the dialog image in x. The example there would be if the bar starts as full. offsetting the location of the dialog image negitively will show depletion of a bar, like an hp bar. If you want to show progress like the GemTD one, just start with a negitive offset that is equal to the size of the dialog box. ^_^
One more thing, there are people on here more qualified than me in this type of problem.
CurrentCount and MaxCount being variables containing the values (e.g. integers) for the items you load, where CurrentCount <= MaxCount. The "minus 8" is room for the border on the Dialog - 4 on either side.
You can get GIMP, free equivalent of photoshop, then grab a .dds plugin for it. After that you can create your own textures and just save them as .dds files. In the editor you can import those files and use them for most texture stuff. Also the good thing about .dds files is that the editor uses its alpha channel so you can make dialogs see through if you wanted.