Homepage Slider

You can configure the slider by going to your WordPress admin and clicking on Posts -> Add New.
This new post should have the category, "slider" (without the quotes). If the slider category doesn't exist yet, just create it by clicking on the "Add New Category" link.
Now click on "Set featured image" for this post you have created and the image you set there will be used to create a single slider image.
You can repeat the process as many times as you like, one Featured Image per post in the "Slider" category.
If you just want a static image, just create one post in the category, "Slider".
Any caption you set for these images will display as a caption on the image.
You can use any resolution you like but 1600px x 700px is recommended.

Homepage About

You can configure this section by going to the WordPress Admin -> Appearance -> Customize -> Homepage Settings

Homepage News

You can configure this section by going to the WordPress Admin -> Appearance -> Customize -> Homepage Settings

Homepage brands

You can configure this section by going to the WordPress Admin -> Appearance -> Customize -> Homepage Settings