About blog images

In the article record you can upload an image. If so, the default settings show a thumbnail on the "article list" page and an enlargement when you click through to the "main article" page. You can amend the sizes of both by going to Articles > Article settings. You will need to re-upload images if you want your new sizes to apply to images you have already uploaded.
Article text
If you prefer the main article page to show enlargements (rather than thumbnails) then go to the main Section record: Content > Sections > Open up the Blog section > Under 'Show images' select 'Show full sizes' from the dropdown.