Yes, in US English:
I show the picture every day. I will show the picture tomorrow. I showed the picture yesterday. I had shown the picture before you told me not to. I was shown the picture on many occasions.
Saying "was showed" or "had showed" is considered a sign of poor English skills. It is a common mistake for children and poorly-educated people.
I have no idea what the convention is in the U.K., perhaps someone else can address that.
I think most English verbs use a X/Xed/Xed pattern. But there are many that are irregular.
I try to exercise every day. I tried to exercise yesterday. I have tried to exercise for years.
And: I shave every day. I shaved yesterday. I was shaved by the barber.
Irregulars include show/showed/shown as above.
Also: I give compliments every day. I gave a compliment yesterday. I have given compliments often.
I think today. I thought yesterday. I have thought about it often.
I eat today. I ate yesterday. I have eaten many meals.