Image Sizes Recommendations for Tuulikki

Recommended Size
Featured ImagesLandscape Mode (Horizontal): 1800 px width (maximum)
Post Slide Images
Promo Box Images
Gallery Images
Sidebar Widgets Images

A Featured Image is a representative picture for your post displayed in different areas of your website such as homepage, post pages, related post areas, recent post widgets, featured area sliders.

Size recommendations! The featured images should not be wider than 1600-1800px, and it’s important to save them for web at 72 dpi, to reduce the loading time of your website. When you see different image sizes in the demos of our theme you don’t worry: WordPress will resize your images automatically.
At this link, you can find how to save your images for web with an efficient online tool (compressor.io).