when shaders are used, looking at the image may cause a glitch where the image has some textures put over it but they're not textures, you can see it in the screenshot below

new information: this only happens when you have a resource pack that has normal and specular maps

Are there versions of Minecraft, Forge, SignPicture and more information on the shaders you are using?

Minecraft 1.12.2, Minecraft Forge 14.23.0.2537, Optifine 1.12.2 HD U C6 and the shaders are Continuum 1.3 Low-Ultra

there's also this glitch i just found, the sign is meant to say "Oh, good! You have SignPictures installed! Carry on :)"