Your sequence settings are 1440x1080 with non square pixels. A still of 800x600 is too small to fit without scaling.
If you crop/resize your stills to 1920x1080 (with square pixels) they will fit perfectly.
I agree with Ann. This ARTICLE goes into more detail on Scaling Still Images, and shows how to Automate to Batch w/ Actions in Photoshop, and also in Photoshop Elements. One can do an entire folder full of images in but moments.
Note: the article was done, when SD was the common Project, i.e. before HD gained popularity. Disregard the Frame Sizes in the early parts, as those pixel x pixel dimensions are for SD. Discussions on HD Projects follows pretty closely, as will some discussion by Todd Kopriva, on how things changed in PrPro, as of CS 5.