First of all, what everyone else is saying is correct, you really should have different assets for print and web.
However, depending on your desired print results you may be able to do what you want. As a general rule, 300PPI at your desired size is an acceptable resolution, it could be double that for high-end products or half that may be acceptable for general home printing. Decide what is appropriate for your use. For example, a 10x8 inch print at 300PPI is 3000x2400 pixels.
Depending on the platform you use for your blog, you should be able to upload at a larger size and have the platform automatically resize and compress the images to a more appropriate size for the blog.
So—decide on the smallest size and resolution you will be able to print at and be happy with, save you images to that size and optimise them enough to be able to upload through your blog platform. Simple.
Note, this is not an ideal solution if you care about image quality to any great extent and you will be pretty stuck if you ever need to print you images any larger.