What is the size in kb of one of the images that failed? The limit seems to be 500k and if you re-sized it to 800px then I can't see it would exceed that but always worth a check.
Does the
Gallery give you any error message when the upload fails?
In PSPX I do the following when convering an image for the web.
Re-size to 800px
Save As...
Select jpg and in the File Req I click on Options
In the Options window I click on Run Optimizer
In the Optimizer window I change the compression setting so that the Compressed Size (under the right-hand preview) is less than 100k.
The I click OK, name the file and click Save.
The DPI value doesn't make any difference to the pixel size of byte size of the image. DPI is just a value used define how large an image would be if printed - the printed size is pixel size / DPI so:
800px @ 72 DPI would print 11 inches
800px @ 300 DPI would print 2 1/3 inches
Applications use the DPI to work out the default size of an image when you import it, etc. So if you imported the above examples into word, the first would fill an A4 page, the second would sit in the corner of the page.