You don't say which blog application you use. Not all blogs are equal. You have no control over the blog visitors internet connection speed, computer performance relative to routine computer maintenance, or the number of applications a visitor has running in the background on their computer. 300 kb should not be to big a file for quick display. However, you can check to see if the blog software allows using a placeholder for your images.
I limit mine on the size of photo not the size of file. All my photo's are limited to 800 pixels wide. On average this comes out to the 300kb but I have some as high as 500kb and as low as 180kb.
I usually shoot for about 200kb give or take. If the file is a good one, you can use a lot of compression before anyone will see any degradation.
I try to stay around 100Kb per file. About 40% of my traffic still comes from IE7 or older believe it or not. Most of those people are likely viewing my site at work, on a POS Windows workstation behind a corporate firewall and bandwidth regulator.
I keep all my images blog and website at 50-100kb to keep things moving quickly. www.sandraadamson.com www.sandraadamson.blogspot.com