In Write the Signs, you are able to add new sprites to the game and then use as fixed image or as parameter in a segment placed layout for certain Text Spots - when they are image spots actually.

All images must be added to [Game appdata folder]/Klyte45Mods/WriteTheSigns/Sprites folder (there's a button in mod options menu that leads it to there). By now, creating subfolders inside it isn't allowed since the mod won't read it recursivelly.

Currently, TIFF, JPG and PNG images are supported. I recommend PNG since it's a loseless format and is the actual format of the sprite atlas itself. The maximum size allowed by now is 400x400. Is recommended to avoid using odd numbers in image dimensions.

Once all textures will be placed in a single image (the Texture Atlas), and the maximum size supported by them is 16384x16384, if all your images have the maximum size, the limit is 1600 imported sprites. It will grow 4x if you use all as 200x200: 6400 images of that size. Seems enought, huh?

Some examples of image quantity loaded:

Images Loaded	Texture atlas size generated x2 (diffuse & alpha)	Assets with images 256x256 equivalency*	Assets with images 512x512 equivalency*	Assets with images 1024x1024 equivalency*
32 images 256x256	512x512 262'144 pixels x2	~3 assets	<1 asset	<1 asset
64 images 256x256	1024x1024 1'048'576 pixels x2	~13 assets	~3 assets	<1 asset
128 images 256x256	2048x2048 4'194'304 pixels x2	~43 assets	~13 assets	~3 assets
300 images 64x64	2048x2048 4'194'304 pixels x2	~43 assets	~13 assets	~3 assets
256 images 100x100	4096x4096 16'777'216 pixels x2	~171 assets	~43 assets	~13 assets
256 images 256x256	4096x4096 16'777'216 pixels x2	~171 assets	~43 assets	~13 assets
256 images 400x400	8192x8192 67'108'864 pixels x2	~683 assets	~171 assets	~43 assets
512 images 256x256	8192x8192 67'108'864 pixels x2	~683 assets	~171 assets	~43 assets
512 images 400x400	16384x16384 268'435'456 pixels x2	~2'731 assets	~683 assets	~171 assets
1600 images 400x400	16384x16384 268'435'456 pixels x2	~2'731 assets	~683 assets	~171 assets

(*) Assets with 3 textures (diffuse, XYS and ACI) in the mentioned size

Resuming: If your system is usually loaded with a lot of assets, you may use as many images you want to. If not, better to use less images or images with lower resolution.