How sets size form same as size images ?

Hellow

I last wrote script to scales images and creats from they thumbnails file.
And I try display this thumbnails file in form.
I want that width and height form will like width and height thumbnails file.
How i make it ?

I try sets this property, but this not work good

FormMiniatura.Width = 520
FormMiniatura.Height = 400
FormMiniatura.IMGminiatury.Width = 500
FormMiniatura.IMGminiatury.Height = 350

My thumbnails file has size 625x425 pixels but is smaller than form and image object. I attachment printscrn what i obtain.

I try use Screen object, but i get 424 error, why ?

nWysokosc = Form1.Height / Screen.TwipsPerPixelY
nSzerokosc = Form1.Width / Screen.TwipsPerPixelX

Regards
Wojtek