Use Paint.net.

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1 Open for example "Crosslow"
2 Ctrl A
3 Edit - Copy

4 Open for example "PzV_G
5 Layers - Add new Layer
6 Edit - Paste
7 Image - Flatten
8 Save
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In this way you can put on the unit what you want: Sideskirts, Tracks, crosses, Flag, machinegun.