The 'data is too big' error refers to the available space inside the .exe file. The tiles are compressed (using a custom algorithm only used by Dangerous Dave) and there is only a fixed amount of space available. When you import a .png file, only the pixels are used (any metadata is ignored) and the image data is converted to the DD format, including compressing it. If, after compression, the data is still too big to fit in the available space, you will get that error. The only solution is to change your graphics so that they compress to a smaller file size, which usually means removing fine detail. Long runs of pixels that are all the same colour compress well, so doing this will result in a smaller file.