a answer to http://stackoverflow.com/questions/19087078/i-want-correct-format-of-this-code

improved_code.m

-(void)btnClicked:(UIButton*)button
{
    NSAssert([imageStr count] == [txtStr count], @"both arrays need to have the same number of items");
    idx += (button == self.nextButton) ? 1 : -1; // if nextButton increment, else decrement
    idx = idx % [textStr count]; // modolo, will keep idx between 0 and count-1
    
    [stgImage1 setImage:[UIImage imageNamed: imageStr[idx]]];
    stgText1.text= [NSString stringWithFormat:@"%@", textStr[idx]]];
 }

original_code.m

-(void)btnClicked:(UIButton*)btn{
    if (btn.tag==1) {
        if (counter < [textStr count]) {
            NSLog(@"%i", counter);            
            [stgImage1 setImage:[UIImage imageNamed:[imageStr objectAtIndex:counter]]];
            stgText1.text= [NSString stringWithFormat:@"%@", [textStr objectAtIndex:counter]];
            counter++;
        }
        if (counter >= [textStr count]) {
            counter = 0;
        }

    } else {
        if (counter>=0) {
            counter--;
            [stgImage1 setImage:[UIImage imageNamed:[imageStr objectAtIndex:counter]]];
            stgText1.text= [NSString stringWithFormat:@"%@", [textStr objectAtIndex:counter]];
        }

        if(counter <= 0) {
            counter = [textStr count];
        }
    }
}

Scrambled Eggs

a answer to http://stackoverflow.com/questions/19087078/i-want-correct-format-of-this-code 
First you should not use view's tag like that. Instead check the identity against a member/property. 
Second I called your count variable idx — as it is a index variable. 
As a pre-condition I check, if both arrays have the same size. Of course it would be much nicer to keep the information together: one NSDictionary per photo and caption. even better: a PhotoAlbumItem that stores both as properties.