PhotoEntry will do a good job of resizing your images for you if necessary, although good results will only be obtained if the uploaded image is substantially bigger than the required dimensions. For example, if a 1440×1080 image is required, submitting a 1740×1305 image may not yield the best result, but submitting a 4200×3150 will be quite acceptable. Clicking on the thumbnail will show the resized image for you to check.
A warning is triggered when one of the image dimensions is greater than the target dimension, but less than 2 x the target dimension. So, for example, for a target width of 1440, it would trigger a warning message if the uploaded width was between 1441 and 2879. This is because the interpolation necessary to downscale under those conditions might lead to visible degradation compared to a downscale from an original with more pixels.
Note that this is not related to any JPEG quality settings – it works purely from the pixel dimensions.
A very common mistake is too resize for the long side. Take the above example. The size requirements specify a maximum width (1440) and a maximum height (1080). This is NOT the same as specifying a maximum long side of 1440! A portrait orientation image would have been sized to 1440 high, which is too big. Most resizing options (e.g. in Lightroom) allow you specify a width and a height which is the correct option, and will retain the aspect ratio (unless you specifically tell it not to).