if you do this often I recommend installing a VTF plug-in for the editing software you prefer. paint.net is what I use but there's also a VTF plug-in for photoshop and gimp. It makes viewing/editing/making crosshairs a lot easier by removing the need to convert through VTFEdit before editing. Also allow you to save images as VTFs directly
If you try to save an image as VTF a window will pop up. Instructions in imgur album
https://imgur.com/a/yF85KqT
Some more tips:
if you replace a VTF crosshair with another you can reload directly with "mat_reloadmaterial thumbnails" in console. Combined with VTF plug-ins this allows you to preview any changes you make to a crosshair immediately in TF2
If you make changes to VMTs or weapon text files you'll have to restart TF2 afaik