It seem that the problem is that the tin (or tin-plated steel, to be
accurate), is being stretched from its flat condition to make a
dome. The paint or ink images would have to be as elastic as the
steel in order to stretch along with it. Not likely, especially if
they are old. In addition, the physical beating of dapping can't help
but it is basically the lack of flexibility and elasticity of the
paint layer. Imagine the metal is like a soft mattress and the paint
is like a sheet of glass lying on top of it. You can drop a bowling
ball (your dapping punch) on the mattress which will deform but not
damage it. The glass, however, cannot go along with this treatment no
matter how much krylon you spray on it.
If you really love these images you might be able to lift them from
the surface by trying various "image transfer" techniques. I'm not
well-versed in this but my wife is constantly stealing images from
all sorts of surfaces by means of heat, solvents or other sly tricks.
Google "image transfer" and see what you get. Then you have to
figure out how to get them back on the surface after dapping. Hmm.
Another thing is that you could xerox or photograph the images.
Xerox ink is heat activated and can also be picked up by certain
solvents so you might be able to xerox the tins' images onto a
deformable material (no, I don't have a clue but this is your
problem, not mine) then transfer the image back onto the metal after
you have finished forming it. In these image-transfer techniques
there is usually some degradation of the image - fine detail gets
lost or blurred. This can be, for some purposes, charming or else it
might be exactly what you don't want. Remember that even if you could
do this, the image will have to be stretched to accomodate to the new
shape of the tinplate and so it will be distorted just as the image
on a balloon distorts when the balloon is inflated.
I knew an old Italian tinsmith who made lovely olive oil cruets and
the like out of emptied 1-gallon olive oil tins with all their lovely
ornate printed surfaces. All of his curved surfaces were simple
curves, not compound curves. In other words he bent them in only one
direction. Like the surface of a cylinder or cone is a simple curved
surface but the surface of a ball or saddle or dome is a compound
curved surface and is a whole different kettle of fish.
Best of luck,
Marty Hykin in Victoria