The fastest and simplest way is to use a comic book archive format.
If you already have the image files, make sure that they are named with the correct alphabetical and numerical order (I.E.
page002.jpg and so on); the program that you will use to read the comic will display your images in alphabetical order.
Then you can simply compress them in a
.zip archive, and rename it's extension from
.cbz. If you prefer, you can also compress the files in a
.rar archive, but in this case you should rename it in
Your ereader should be able to read one of these formats.
Alternatively, you can try PDFtk, it is a nice tool for manipulating PDF in various ways; it is cross-platform (hence you can find it also in the Ubuntu repositories) and can be used to create a PDF from the source images.
I've always used it from Command Line, but on openSUSE repositories I found at least a couple of graphical interfaces, the packages are named
pdfchain; I suppose that these should be available on Ubuntu as well, try to look for them (bear in mind that on different distros, the package names could be a little different).