how important are images in blogs to seo?

Images are very important but they are not essential for every single page you create. If you have a theme on your site that doesn't require an image thumbnail for every post in the taxonomy pages and menus, then you won't have to worry about the astetic side of things. 

The big thing with images is that Google seems to love them. The alt tag is a really valuable place for you to be able to get some target keywords added to better describe your post. Does this mean you should include any old image just to tick the box? Some would say yes, but its hard to know. If you are going to include images that negatively impact the user experience, it is best to leave them out. For platforms like wordpress, an image is normally displayed at the top of the post. A good theme will also ensure that the alt tag is used so that you can hit your target keyword. These images tend not to get in the way. It isn't much harm to include them.

I understand that it is hard at times to find a relevant image, but you should try. I would advise you do your very best to make sure every post has one. Images are very important to blogs when it comes to SEO. If you need to just find some generic enough image and put the title of the post over the image, this is fine. It is easy to do and you won't run into any legal trouble with it.