The fact that some of the images are the right size appears to be pure luck, as the originals of those images are proportined width:height = 8:6.
The div that contains your images is 640px wide by 500px high (with padding 10px) which means that the maximum width image it will currently accept is 620px, and maximum height will be 480px, so using the 782p setting will cause you problems when the width:height ratio is (>8):6.
Your gallery is set up to fit the images proportionally (as opposed to cropping them), so it appears that it is fitting the images first by height, and then adjusting the width to be proportional to that. It is coincidental (I believe) that for most of the images, when the width is adjusted to be proportional to the height, that the height:width ratio happens to be appropriate for the space provided.
If you look at image #6 in the sequence (Flower border) you will see the correct height again, but in this case proportioning the image makes it narrower than the others.
Your choice is to either follow Jason's advice above (adding 620px width as an option), or manually adjusting the height:width ratio (to 8:6) of the original (too wide) images before uploading them.
Edit: PS - nice design work on the site!