N.B. I'm first going to explain how Tapku currently draws its MonthView marks, and then I'll propose a way to change it.
Tapku doesn't actually set those marks as images; it sets them as strings! Search TKCalendarMonthView for •. The marks are set in two different places in TKCalendarMonthView: First, in the drawTileInRect:day:mark:font... method, which is called on each tile individually in the drawRect method. Second, with the property 'dot', which is applied to the user's 'selected' cell, which has a different text color, etc. and thus needs to have its own properties adjusted.
To set your own images, you'll have to modify Tapku in those two places (not terribly difficult; it's a pretty accessible project). So, instead of setting the cell's text to •, you'll have to set its image to an image you provide.
Providing this image could be done in a few different ways. The most straightforward will be to redo Tapku's concept of the 'marks' array (set by the delegate). Instead of creating an array of integers, perhaps you could create an array of UIImages. Still, you need to have a way to tell the code "no image"--maybe have a blank image and just apply it to cells by default?
Let me know if you need any clarification.