Photograph size guidelines

I have just transferred my blog to WordPress, but cannot load the images so that they are clear.

I take very high res images, which I have resized to low res. On Blogger they loaded with no change. Now as I am trying to upload the missing images they are very indistinct and blurry.

Please help as I am at my wits end with this. What are your specifications for this – so I can try to match it to WordPress.