didn't i like.. already tell you just below?
lemme make it fuller tho..
the tentacles those infested dudes have, they are too sharp and too elongated, make them look more like an angling hook, ya know, for fishing. (maybe lill less round tho)
second pair of those, just mirrored down, and both of those should start from the lower/mid back. also make sure meshes don't clip with each other, which they do on where your tentacle-spikes start
second thing, take a lill better look at how the portrait of the infested marine looks and works.. and keep tinkering on it
and third, why would you even need a custom texture when you can totally use a zergling or hydralisk textures from war3. they're already zergy-colored
as for paint editor.. uh.. everyone uses Photoshop.. or 'Paint Tool Sai'
there's also gimp, but i don't want to recommend it to you <_< i didn't 'get it'