If you want to make change to content of your theme, css or images, you are doing these changes under app/design folder.

Magento is looking for files under pub/static folder, that means you must deploy this static content there.

MyCompany is name of your company mytheme is name of your theme

You need to remove files from pub/static folder first, Magento will not remove them. So if you upload new image under app/design then remove first images from pub/static and then run command for deploying static content.

php bin/magento setup:static-content:deploy en_GB --theme Magento/backend
php bin/magento setup:static-content:deploy en_GB --theme MyCompany/mytheme

If you install some new module, you need then recompile files.

php bin/magento setup:upgrade
bin/magento setup:di:compile
sudo chmod 777 var -R
sudo chmod 777 pub -R