I've been mucking about with JP LeBreton's edscii http://vectorpoem.com/edscii/ . there's also a successor called playscii http://vectorpoem.com/playscii/ . I've been auto converting images with a custom font.
I like how you can see the original image and the text characters at the same time in the default resolution. the program can convert images with enough text characters and colour range that a reasonable representation of the original image comes out of the conversion.
I have some ideas for things I would like to try with this. the artwork here is from Clicky Clicks Dungeon. but i've been chucking all sorts of things at it.
I have worked out how to get the output of the program to display on a spectrum. the colour pallet and the low number of characters make it harder to tell what the images are of though. i guess things could be improved by manually editing the characters after the auto conversion.
my previous efforts on the zx spectrum used a combo of lines drawn between points and text characters. if I want to use this technique for the next spectrum thing I do it may require a bit of work to help make the images readable at this resolution. but they have a weird quality that I like so I'm going to keep experimenting.
I might be able to do a bit of compression to keep the size of the images down as I think they are generally a little bit bigger than the technique I was using before. I don't want to spend too long on changing the code I already have, and want to get on with the successor to Clicky Click. I have a bunch of candidates which might be the one!
if your looking to create a custom font on the spectrum you might try FZX editor which is part of zx modules found at http://www.zx-modules.de/ . it can spit out a bunch of asm. although your new font is mostly a list of defb statements so there are other ways. I decided to make do with uppercase letters and used the lowercase letters as space for the graphic characters.