The Source Monitor will always show a full image, regardless of its size, if the viewer is set to Fit. The Program Monitor viewable area represents the pixel dimensions of the sequence you're using; if you set this viewer to Fit, you'll see the full output area, regardless of sequence dimensions.
Your images are larger than the sequence dimensions. The sequence needs to match the images (not desirable, probably) or the images need to match the sequence (a little more desirable). The third option is to scale the images down in Premiere Pro to reveal more of the image in the Program Monitor and therefore, the sequence.
Read here for information on scaling: Motion: position, scale, and rotate a clip
There are a couple of ways to handle this.
Basically, the Images do not match up to the Frame Size of the Sequence, so Scaling must take place.
One can do Scale to Default Frame Size, they can use the Fixed Effect>Motion>Scale, or they can Scale in an Image-editing program, like Photoshop, where the process could be automated via a simple Action, and Automate to Batch. This ARTICLE will give you some tips. Note: it was written, when most Projects were SD, so ignore those pixel x pixel dimensions, as more Projects are now HD. The processes will be the same, but the numbers will be larger.
As of CS 5 (and also 5.5), the Scaling algorithms have been greatly improved, but there is still the matter of the processing overhead, with overly-large still images. If one has many, then doing the Scaling in PS would be advised.