Thumbnail vs Lightbox

Hi my friends , i am building a website where i am going to post a lot of photos.
What i need is a solution(plugin) to post the images on the size i want inside the post ( i did this by creating the thumbnails on the size i want) , but when i use plugins like LIGHTBOX etc , when i click on the images posted it shows me the size of the thumbnail not the original size of the photo.
pLEASE can you help me ,
it took me a month trying on this but i couldnt find a solution.
Thank you

[No bumping. If it’s that urgent, consider hiring someone.]