Generally, it’s good to have an established convention for photo size and placement within a post or page that remains consistent throughout your website.
Dimensions — the most important dimension when placing images is width, as it can affect other elements in your layout. Once a convention for image width is established, it’s best to be consistent throughout your site. Height can vary to some degree, but a consistent look is best and too much variation can also affect your layout.
If you have your site built by a web developer, the font sizes, line spacing (leading) and other attributes should already be established. It’s best to stick with these conventions to maintain a clean, consistent appearance for your content.
Character Spacing — computer fonts are proportionally spaced. Unlike typewriters with mono (fixed) spacing, computer fonts (with a few exceptions) adjust spacing automatically, based on the context in which they are placed. It is for this reason, that a single space after punctuation at the end of a sentence is appropriate. Two spaces at the end of a sentence is a convention that was established with mono-spaced typewriter fonts. The result of using two spaces with proportional fonts is a river of white running down the page.