What I used to do for text in signatures long ago, was to find two contrasting colors or shades within the image, set one as basecolor for the font (use the color picker!), and the other as a very thin outline. Usually, I picked the lighter shade as a basecolor, with the darker shade as an outline, but occasionally the other way round worked out better.

That way, you usually get two colors that won't clash with the image, while the result is generally fairly readable because either the font or it's outline will usually have a contrasting shade against the background.

So in for example the Toad Lilly siganture, I'd probably pick a very pale yellow from her unevolved garter as the main color, and a very dark shade of purple from the bottom of her evolved dress as the outline color.
The pale purple from her evolved stocking as basecolor with the orange from that piece of her evolved dress as an outline color might work too, by the way.