Do you know what favicons and app icons are? On the one hand, favicons are the little icons which are shown in every browser tab where a website is opened and a favion is set up properly. Here is an example:
They are truly important because they are also visible at bookmarks, thus the website is easier to identify. On the other hand, app icons are shown at mobile devices like iOS and Android if you launch an application.
Given that you already have a logo in .jpg or .png format, just use this online favicon generator. This is a very simple and convenient way to generate all necessary little icons:
Simply upload your logo in .jpg or .png format. After that, you can choose between only generating the 16×16 favicon or generating icons for all devices. The image dimensions can also be maintained.
After pressing the “Create Favicon” button everything gets generated and you can download the .zip file containing your icons. You can use these images by uploading it to your web application like WordPress. Mostly every theme offers the option to upload those icons in the theme options.