What would be a good graphics program for designing decals, for shields and such?

I would recommend a vector graphics editor, like Inkscape or Adobe Illustrator.

Perhaps photoshop.

If you don't want to do vector, GIMP is awesome and free; it's what I use to do decals and all my graphics work in anyway. There's a lot of good tutorials on the internet, and many PS tuts can be adapted easily to GIMP. GIMP has a lot of plug-ins and filters as well as brushes and patterns available, and if you find PS brushes you like you can use them in GIMP as well. GIMP also opens .psd, PS' default filetype, so you can download and edit them.

Inkscape is a good free vector program, I've never used it though.

I find that GIMP works magnificently as well. I haven't tried Inkscape yet either, I'll look into that.

Essentially I need to find a program that does circles, so I can start designing me some Greek shield designs of my own for my ever growing army.

In GIMP: Use ellipse selection tool, hold down Shift, then drag your selected color into the shape and fill it.

Tbh, you can do the same thing in MS Paint.

been trying but for some reason the coordinating to do a good circle eludes me.

You can find most shield style templates on Eurobricks, which you can the design easily with the correct dimensions.

have the templates, its the inner circles and such need help making.