Can nice pictures be a deciding factor? Let's say Paper A was recently accepted by a fairly prestigious journal (the top in my subfield).  Now, there is a new Paper B, half of which gives a more detailed description of an object in Paper A for pedagogical reasons. This half of Paper B  may not technically be new, but there are lots of pretty pictures in it (not merely pretty, of course, they accurately illustrate the particular object of interest). Much effort went into creating the illustrations, and this will probably be apparent to the reviewer.

What are the odds that the same journal would accept Paper B, if the first half of Paper B would not be accepted on its own merits?  To what extent can an editor's or referee's judgement be affected by nice colorful images?
Good illustrations can have disproportionate impact; this is true. Reviewers often seem to be swayed (I'm uncomfortable with 'fooled' here) by new or interesting looking images. This is generally true, and papers like this often are frequently cited, so that is good for the journal. Effort invested in this area is likely to be rewarded, though it may not substitute for substance.

However, as to your specific question about the same journal accepting it, that may be very difficult to judge. It depends on what the other half of paper B is. If the same explanation is applied to some problem, I should think it will be well accepted. If it's a restatement of a previous paper, unless it contains a contrary viewpoint, I think it's unlikely to pass muster. This is assuming, of course, that the reviewer/editor connect it to the previous paper. If they don't, then it's a fresh paper to them.