Implications of Supertype and Subtype

Question

Is it bad to implement supertype and subtype to the entire data in a database? I need some advice on this before moving to this direction...

For instance,

I have these tables as objects and they are related,

users
pages
images

entities table as the supertype

entity_id     entity_type
1             page
2             page
3             user
4             user
5             image
6             image

users table

user_id     entity_id
1           3
2           4

pages table

page_id     entity_id
1           1
2           2

images table

image_id     entity_id
1            5
2            6

here is the table to map images table with entities table because some images belong to certain page (maybe to blog posts, etc in the future),

map_entity_image table

entity_id    image_id
1            1
1            2

so, I will insert a row into the entities table when I have a page, an image, an user, etc to be created.

in the end of the day the rows in this tables will increase in a great numbers. so my worry is that can it cop with large numbers of rows? will this database gets slow and slower in time?

after all, are these a bad structure?

or maybe I am doing supertype/ subtype incorrectly?

edit:

I think the entity should have these data only,

entity_id     entity_type
1             page
2             page

unless I want to attach images to users, etc. then it should be like this,

entity_id     entity_type
1             page
2             page
3             user
4             user

maybe I am wrong...

EDIT:

so this is the query how I find out how many images attached to the page id 1,

SELECT E.*, P.*, X.*,C.*
FROM entities E

LEFT JOIN pages P ON (P.entity_id = E.entity_id)

LEFT JOIN map_entities_images X ON (X.entity_id = E.entity_id)

LEFT JOIN images C ON (C.image_id = X.image_id)
WHERE P.page_id = 1

returns 2 images.