Nobody needs a professional camera to show real work well. Phone cameras have been good enough for years. Most job-site photos that look bad fail for the same handful of avoidable reasons, all fixable in a few seconds before pressing the shutter.
Shoot in real light, not overhead sun or dark rooms
Direct overhead sunlight creates harsh shadows that hide detail, and a dark interior with the phone's flash on produces flat, washed-out results. The best light is usually early morning, late afternoon, or a bright but overcast day outdoors, and by a window, not the overhead light, indoors.
Clean the frame before you shoot
A finished job surrounded by tools, trash, or a cluttered truck bed reads as unfinished, even when the actual work is done. Thirty seconds of tidying the frame before the photo makes a bigger difference than almost anything else on this list.
Get close enough that detail is visible
A photo taken from across the yard shows a shape, not the craftsmanship. Move closer, and take a few detail shots of the specific work (the seam, the finish, the fixture) in addition to the wide view.
Hold the phone level
A tilted horizon or a leaning wall is a small thing that reads as sloppy without the viewer quite knowing why. Most phones have a built-in level grid in the camera settings; turning it on costs nothing.
Shoot both before and after, from the same angle
A before-and-after pair is only convincing if it's clearly the same spot. Standing in the same position for both shots does more for credibility than either photo alone.
Don't over-edit
A heavy filter or oversaturated edit can make a real photo look like a stock photo, which undermines the entire point of using real work in the first place. See why stock photos hurt a local business more than they help.
Questions
What resolution should I shoot at?
Whatever the phone's default camera resolution is; modern phones default to more than enough detail for a website. The bigger issue is usually file size once it reaches the website. See image file size and website speed.
Should I shoot in portrait or landscape?
Landscape (horizontal) for most website uses, since that matches how photo galleries and hero images are typically laid out. Portrait shots are fine for detail close-ups used individually.
How many photos should I take per job?
More than you'll use. A handful of extra angles from every job costs nothing and means you're never stuck using a mediocre photo because it was the only one taken.