1) What do you mean by "stretch across my blog page"? You have a sidebar, so the images aren't supposed to stretch across the page, they're supposed to stretch across the post column.
2) As timethief pointed out, the files you've been uploading are huge: the ones I checked are 1.9 or 2.3 MB each, 2853 or 3872 px wide - while the maximum for the theme you're using is 560 px. When you upload such huge files you simply waste your storage space and increase the loading time of your pages. For future posts at least, the ideal thing to do would be to use an image editing application or an online tool to downsize copies of your images to a width of 560px and upload/insert those copies (selecting the Full Size option).
3) There's no point making the images clickable - see here:
4) Endless scrolling down isn't very user friendly. You can use the read-more tag to display only the beginning of each post on your main page - see here: