For photographs it is best to leave them uncompressed and the default size and in the jpeg (or jpg) file type. GIF file types can also be used.
For a good quality photo on laptops & mobile displays 1000pixels x 700pixels is fine, having 15 high quality images for example may cause the website to take longer to load. However, this is unlikely to be noticeable unless the site is being loaded over a slow connection such as mobile data.
Compressing the images will cause the website to load faster and would be more effective than resizing the images however the images may have a noticeable reduction in the quality of the image.
When adding Images to a page you should click OK for every photo.
Specifications for Images:
940x150 pixels for Homepage Banner.
620x304 pixels for News Images (General landscape images on a page as well).
300x250 pixels for Boxed Images.
300x120 pixels for Half-block Images.
Sponsors images can be anywhere between maximum width of 150 pixels and maximum height of 100 pixels. Keep all sponsor images roughly the same dimensions.
630x222 pixels for the Animated Banner Images.
75x113 pixels for Athlete Profile Images.
For The Club sites -
For the large image (Welcome Image) underneath the navigation banner - 320x240 pixels.
News Thumbnails - As long you stick to roughly 1:1 aspect ratio it should be okay. (This can be anywhere between 0.75 to 1 if need be, and it should still look okay. Try not to upload images past 1000x1000, as they tend to get rather large and make the page take some time to load).
News Images - Width shouldn't exceed 620 pixels and height shouldn't really exceed past 1500+ pixels for loading purposes.