One of the reasons some blog's take a long time to load, is either a huge photograph at the top of it or a particular post is photo heavy and the images have been loaded in the size they were taken, which can be very large. My camera for instance produces photographs around 45cm x 25cm, way to big for a blog. Another problem with adding them so large is that it eats up your free storage space.
If possible try to adjust your photographs to about 15cm x 10cm (landscape) or the other way around if portrait. If you don't have a programme such as Photoshop Elements, Photoshop or other such software that came with your computer, don't despair, there are free sites on the web for you to try, such as these:
plus many more. I haven't tried them out but have tried to chose ones where you don't, at first glance, have to register or give out your email!
Anyhow, onto the tutorial, putting your photograph into your blog header.
First of all, go to your home page, the one where you can read new posts by those you follow etc. Towards the top you will find this box, click on the little downward facing arrow:
It will bring you to a new screen, on the left of which is this area. Now click Layout:
then go and find the picture you want. Click on it then at the bottom right of your screen, click open. This screen appears, which is very similar to the flower one before but now your new image is showing
I always have the top button pressed which is show and says "Behind title and description", but you could click either of the other two:
This will bring you back to your blog layout screen, the top right of which is this:
I hope this all makes sense to you and that the photo resizing sites I have given, will be useful!